1. Technical Field
The present invention relates to a method of measuring a characteristic of a test part and also to apparatus for carrying out such a measurement by use of a sensor including a coil, the impedance of which is influenced by eddy currents induced in the test part.
The method and apparatus to be described allow non-destructive measurement of a variety of characteristics of test parts, for example the thickness of an electrically-conductive test part or of an electrically-conductive coating applied to the test part.
2. Prior Art
Existing eddy current sensors include a coil to which an alternating current of predetermined frequency is applied. The coil is also connected to a control apparatus for measuring the impedance of the sensor when the sensor is in contact with or adjacent to a test part. Eddy currents induced in the test part effect the impedance of the coil. The magnitude of the eddy currents generated depends on the nature of the test part, and also on the proximity of the sensor to the test part in a sophisticated manner which does not need to be fully understood for implementation of the invention.
The impedance of the coil is a vector which can be completely defined by measuring two orthogonal components of the impedance. Theoretically the impedance vector can be defined as a purely resistive component and as an orthogonal, purely reactive, component. The reactive component is frequency dependent and includes induction and capacitance effects. Known control apparatus for measuring two orthogonal components of the impedance vector do not, in general, allow recognition of the resistive and reactive components but, rather, provide for measurement of two, arbitrary orthogonal components. It is normally possible to adjust the control apparatus so that it measures components of the impedance in any two arbitrary orthogonal directions. Thus, the two reference axes of a control apparatus for measuring two orthogonal components may readily be adjusted in phase as required. It may also be possible to adjust the origin of the orthogonal reference axes in a translatory manner.
In the following discussion reference is made to Horizontal and Vertical-components of impedance. It will be appreciated that these are arbitrary, but variable, orthogonal directions.
If a sensor coil is supplied with an alternating frequency signal and the horizontal and vertical components of its impedance are measured and supplied to the X and Y inputs of an oscilloscope, the impedance measured is visualised on the oscilloscope screen as a dot. If the sensor coil is then brought towards an electrically conductive test part the point on the oscilloscope screen which represents the impedance traces a path on the screen. The trace produced on the oscilloscope screen as a result of the point traversing its path in time will be referred to hereinafter as the lift-off curve. The curve extends between two points. A first end point represents the impedance of the coil in space when it is not influenced at all by the presence of the test part. The other end point is the value of the impedance when the sensor coil is in contact with the test part.
Under predetermined operational conditions it is found that the length of this curve between the two points and its orientation relative to the first end point representing the impedance of the coil in space is a function of characteristics of the test part, and the distance between the sensor and the test part. For example, if the test part carries a non-electrically conductive coating on its surface then the resulting clearance between the sensor and the test part results in a curve of a length which varies as a function of the thickness of the layer. A layer of air between the test part and the sensor has a similar effect.
Other characteristics influence the location of the curve. For example, the magnetic permeability of the test part, its conductivity, the presence of fissures or defects in the material of the test part and the thickness of the test part all effect the location of the curve. In the case of determining the thickness of the test part, in particular, the influence is only measurable if the thickness of the test part is of the same order of magnitude as the depth of penetration of the eddy currents into the test part.
Hitherto, it has been proposed to use this type of sensor by bringing the sensor into contact with the test part and comparing the value of at least one component of the impedance measured at this point with a reference value. This technique has numerous disadvantages. Since the materials of which the sensor is made are generally fragile, rapid wear is apparent and a deterioration in the coil follows repeated shocks on the sensor. The pressure exerted at the end of the sensor can give rise to parasitic effects which falsify the measurements. The sensor, which is heated by the Joule effect is cooled by thermal exchange with the test part which is at ambient temperature. This temperature variation gives rise to parasitic effects. Further, if the axis of the sensor coil is not truly orthogonal to the test part then there will be an effective clearance between the coil and the test part which, in turn, effects the impedance measured.
It is possible to reduce certain of these disadvantages by adhering a strip of synthetic material to the tip of the sensor in order to protect it. Alternatively, the sensor coil can be displaced slightly to the rear of the tip of the sensor. However, these solutions result in a variability of the response of different sensors because of differences in the displacement of the coil from the sensor tip or the thickness of the protective synthetic material. This can falsify the measurements. With this type of solution recalibration and sensitive corrections are normally required.
DE-A No. 3 324 444 describes an eddy current sensor which is capable of effecting measurements without contact between the sensor and the test part. In this case, a differential sensor is used which has two coils which perform the double function of measuring the lift-off curve on the one hand and also monitoring a characteristic of the test part on the other hand. Such an apparatus allows measurement at any time of the error due to variation in the lift-off curve and uses a complex calculation function in order to correct this error during scanning of the test part.